1. We tested the mechanism of secretion in bovine adrenomedullary cells. We found that catecholamine release correlates best with calcium influx and less with free intracellular calcium levels. Furthermore we found conditions in which increases in intracellular calcium levels were not accompanied by release. We conclude that calcium in specific sites within the calcium channel is essential for initiation of the secretory process. We also found that high K+ and nicotine activate two different calcium channels which are functionally distinct. 2. We have shown that there is no difference in resting intracellular calcium levels in platelets of SHR and WKY rats when the cells are in the basal state or after various perturbations. Blockade of Na-K pump with Ouabain does not cause permanent change in intracellular calcium levels. Multiple effective mechanisms buffer quickly any changes in intracellular calcium induced by blockade of the Na-K pump. These results refute Blaustein's hypothesis which suggests that in hypertension intracellular calcium levels are elevated due to a circulating inhibitor of Na-K ATP'ase.